Red cell mass, derived from plasma volume and haematocrit, was calculated serially in 45 healthy women during pregnancy and 4 to 6 months after delivery. Compared to the non-pregnant, postpartum value there was a reduction of approximately 100 ml at 12 weeks gestation. An increase in red cell mass occurred thereafter: at 36 weeks gestation values 180 ml greater than the non-pregnant were found. Iron and folic acid supplements almost doubled this increment to 349 ml of red cells.